Keratoconus (KC) is a progressive disorder with conical deformity of the cornea. It is characterized by corneal thinning that induces irregular astigmatism, myopia, and protrusion, resulting in mild to marked impairment in the quality of vision [1]. It is usually bilateral; the apex of the cone is usually centered just below the visual axis [2]. It is the most common primary ectasia and usually occurs in the second decade of life and affects both sexes and all ethnicities. The estimated prevalence in the general population is 54/100 000 [3].

The etiology of KC is unknown and most likely multifactorial, and includes keratocyte apoptosis [4], biochemical abnormalities [5], changes in collagen orientation and distribution [6], genetics and hereditary [7], association with genetic disorders such as Down's and Marfan syndromes [8], and the microtrauma associated with eye rubbing [2].

Pentacam is an elevation-based diagnostic imaging system that is considered highly accurate in detecting KC and KC suspects [9]. We used a new surgical system referred to as the corneal intrastromal implantation system (CISIS), in which the MyoRing flexible full-ring implant is inserted into a corneal pocket using a high-precession microkeratome (PocketMaker microkeratome); this is a new type of continuous ring implant (MyoRing) having a-priori conflicting features, such as rigidity and flexibility, allowing safe, easy, and effective treatment of myopia, KC, and post-LASIK keratectasia [10].

Aim of the work

The aim of this study was to evaluate the efficacy and safety of implanting a complete ring (MyoRing) into a corneal pocket, known as the corneal intrastromal implantation system, for the treatment of KC.

Patients and methods

This interventional, nonrandomized prospective clinical trial was conducted at Benha University Hospital and the Egyptian Academy of Ophthalmology during the period from January 2012 to April 2013. The study included 23 eyes of 23 patients (12 male and 11 female) with KC. The mean age was 21.7 ± 10.9 years (range 13-48 years). Exclusion criteria included evidence of corneal opacity, history of corneal surgery, and corneal thickness less than 360 μm. Informed consent was obtained from all patients after thorough explanation of the treatment approach and surgical procedures with their possible side effects and potential complications. The study was approved by the research ethics committee at the Faculty of Medicine, Benha University. A patient was excluded from the study because of intraoperative suction release during pocket formation, with subsequent failure of MyoRing (Dioptex GmbH, Australia) implantation.

The surgical procedure includes creation of an almost entirely closed intracorneal pocket that is 9 mm in diameter and 300 μm in depth through a small incision of ˜3 mm made using a PocketMaker microkeratome. The device consists of a suction ring [Figure 1], a transparent disposable applanator, which defines the cutting depth, and a microvibrating diamond blade, with its tip following a circular curve of 9 mm in diameter without penetrating the cornea along this path. The MyoRing intracorneal implant is inserted into the corneal pocket through the small incision tunnel and is centered on the corneal reflex. The procedure is self-sealing and no suturing is required. After a minimum of 1 week, refractive data and Pentacam images were obtained. The criterion to perform ring adjustment as an enhancement procedure included increased corneal astigmatism greater than 4 D; the cone can be further 'pushed' out of the center to the degree of patient satisfaction. The pocket was reopened using a spatula and the implant was shifted 0.5 mm toward the apex using of a forceps. This step can be repeated as long as the best possible refractive result is achieved. Any surgical complication was reported. Postoperative follow-up visits were at 1 day, 1 week, and 1, 3, and 6 months postoperatively, with measurement of UCVA and BCVA, Pentacam corneal imaging, and slit-lamp examination.

In this clinical trial study visual acuity and Pentacam topography outcomes were analyzed in patients diagnosed with KC, with an analysis of the postoperative time course of MyoRing-mediated clinical changes over 6 months. There was a high statistically significant improvement in the UCVA and BCVA when comparing the preoperative with the postoperative values (P < 0.001). There was a high statistically significant improvement in the mean K1 , K2 , and Km values postoperatively (P < 0.01). There were no statistically significant changes in the mean corneal astigmatism and thinnest location values postoperatively (P > 0.05).

In the current study [Table 1], there was a high statistically significant improvement in the mean UCVA from 0.12 ± 0.17 to 0.5 ± 0.24 at 6 months postoperatively (P < 0.001). There was a significant improvement in UCVA by 4 or higher decimal lines. In addition, there was also a high statistically significant improvement in the mean BCVA from 0.3 ± 0.143 preoperatively to 0.7 ± 0.22 at 6 months postoperatively (P < 0.001). There were significant improvements in BCVA by 5 ≥ 4 decimal lines. The mean K1 value decreased from 48.45 ± 3.79 to 42.56 ± 3.17 postoperatively, with a reduction by about 6.0 D in the first week and minimal changes later on; in addition, the mean K2 value decreased from 54.6.45 ± 3.94 preoperatively to 49.07 ± 3.33 postoperatively, with a reduction by 5.0 D or higher in the first week and minimal increase thereafter. The mean Km value decreased from 51.31 ± 3.56 preoperatively to 45.79 ± 2.97 postoperatively, with a reduction by 6.0 D or higher in the first week and slight increase thereafter. The mean corneal astigmatism improved from 6.01 ± 3.13 to 5.93 ± 2.45 D, and the thinnest corneal location changed from 427.6 ± 39.62 to 422.7 ± 43.83, both at 6 months postoperatively, with no statistical significance (P > 0.05).

Four patients with severe KC underwent position adjustment of the implant (two eyes at 1 week, one each at 1 month and 3 months postoperatively), with 0.5 mm displacement toward the apex of the cone within the pocket. This achieved marked visual, optical, and corneal astigmatic improvement, with no recorded drawbacks or complications. A patient was excluded from the study because of intraoperative suction release during pocket formation with subsequent failure of MyoRing implantation. Ring extrusion was reported in a case 9 months postoperatively due to continuous eye rubbing following vernal keratoconjunctivitis [Figure 1], [Figure 2][Figure 3],[Figure 4].

Figure 2: Linear representation of the mean K1, K2, and Km values throughout the study period.

Figure 4: Linear representation of the mean thinnest location values throughout the study period. Pentacam photos of four patients before and after MyoRing implantation. Pentacam Photos of four patients, two before and two after MyoRing implantation.

KC is characterized by progressive thinning of the cornea and is accompanied by ectasia [12]. Abnormalities in the arrangement of collagen fibril layers may be considered an ultrastructural basis for the lack of biomechanical stability [13]. Adding volume to the peripheral cornea by implanting ring segments into circular corneal tunnels may improve visual acuity and reduce central corneal steepening in KC [14]. Such treatment may delay or eliminate the need for corneal grafting and improve the quality of life of affected patients.

In the current study, we used Pentacam because it is considered highly accurate in detecting KC and measuring its corneal optical parameters [9].

As the thinnest area of the cornea is involved in the formation of a pocket at a cutting depth of 300 μm, we excluded KC corneas that were less than 360 μm in thickness from the study, whereas Jabbarvand et al. [15] evaluated the clinical outcomes of CISIS (MyoRing) implantation at two different depths of 250 and 300 μm using femtosecond laser technology. They reported that no differences were observed in visual and refractive outcomes, keratometry, corneal biomechanical characteristics, and higher order aberrations at the 1-year postoperative follow-up. They concluded that an implantation depth of 250 μm has comparable outcomes with the previously applied 300-μm implantation depth. This may be appropriate for selected cases of KC with lower pachymetry.

We used the mechanical Dioptex PocketMaker to create the pocket. Daxer et al.[11] compared the same technique with the femtosecond laser technique (Ziemer LDV) as two different methods of pocket creation for MyoRing implantation. Both groups did not show any statistically significant difference, neither in the KC severity nor in the results. They claimed that both the femtosecond laser technique and mechanical corneal pockets gave the same results in treatment of KC with MyoRing. In the current study, a patient was excluded from ring implantation because of intraoperative suction release during pocket formation.

An important question is whether the creation of a corneal pocket reduces the biomechanical stability of the cornea. As long as cutting is performed in the direction in which the tension inside the tissue unfolds, namely parallel to the collagen fibrils, impairment of the biomechanical stability is not expected. If cutting is performed perpendicular to the tensile forces (as in LASIK, in which the collagen fibrils are intersected along the circumference of the flap), the entire flap tissue no longer fulfills its stabilizing function. This is why LASIK leads to a reduction in biomechanical stability, which in turn may result in keratectasia. Such problems should not occur when using CISIS, as the pocket is almost entirely closed along the circumference, and the only existing cut unfolds more or less parallel to the tensile forces (longitudinal axis of the collagen fibrils); the collagen fibrils are cut through (perpendicular) only in the area of the small incision tunnel.

A smoother corneal surface is an additional advantage of the pocket, as it is not covered by a flap but is closed almost along the entire circumference, together with the inserted full-ring implant keeping higher order aberrations to a minimum [16]. Moreover stability of the implant is due to the transcorneal pressure, which is 'trapped' between the anterior and posterior lamellae, such that displacement of the implant is unlikely to occur. The access to the pocket is self-sealing and does not require suturing. As this is also the case when an implant is introduced between the lamellae, it would be more appropriate to speak of a 'virtual cleft' rather than a pocket. However, in the current work, the MyoRing was extruded in an 18-year-old patient at 9 months postoperatively because of continuous vigorous eye rubbing following vernal keratoconjunctivitis.

The corneal pockets were created at a 300-μm depth, with a diameter of 9 mm. The nomogram for the selection of the appropriate implant diameter and thickness depends only upon the central average K-reading (average Sim K) and is optimized for the Middle Eastern population [11]. In contrast to intracorneal ring segment nomograms, the CISIS nomogram is very simple and does not consider either cone type or cone location or astigmatic axis, etc.

In agreement with the findings of Daxer et al.[17], we found CISIS to be an effective treatment procedure for KC. MyoRing achieved a high statistically significant improvement in both UCVA and BCVA when comparing the baseline values with all postoperative values. It significantly improved corneal topography with a high statistically significant improvement in the mean K1 , K2 , and Km values (P < 0.001), together with nonsignificant changes in the mean corneal astigmatism and thinnest location values.

Few intraoperative and postoperative complications were reported in the current study. However, clinical significance was not sufficient to warrant repositioning, replacement, or removal of the implant. Jabbarvand et al.[18] also reported that the MyoRing offers potential advantages over other existing lines of treatment, being a reversible, minimally invasive, quick, safe, and easy procedure, with no major complications during or after surgery. They explanted the MyoRing in four eyes (4%). The refraction, visual acuity, and corneal topography returned to their preoperative statuses 1 month later in all four eyes.

The CISIS technique allows simple adjustment of the implant position toward optimal placement at any stage after implantation [19]. Because the shape of the cornea is highly irregular in advanced KC, there is no existing theory or method to predict the optimal implant position in a given case before surgery. Therefore, it appears necessary to consider adjustment of the position of an implant after its insertion to achieve optimal results. In the current study, four patients with severe KC underwent position adjustment of the implant at variable postoperative time points by ring displacement toward the apex of the cone. This achieved marked visual, optical, and corneal astigmatic improvement with no recorded drawbacks or complications. Changing the implant position by merely 0.5 mm within the pocket produced a much better result. In conventional corneal implant surgery, the ring segments are closely associated with the position of the circular corneal tunnel, and thus the position of a ring segment may only be changed along the course of the tunnel [20]. The only way to change the position of an implant relative to the center after its insertion is by introducing a flexible full-ring implant into a corneal pocket, as in the technique used in this study. This method also allows positioning of the implant independent of the positioning of the microkeratome.

In the current study, outcomes of visual acuity and Pentacam topography were analyzed in KC patients, with analysis of the postoperative time course of MyoRing-mediated clinical changes over ± 6 months, which is considered a relatively short duration for such a progressive disease [17], and the question of whether CISIS has the potential to improve KC treatment still needs long-term results from a larger number of patients to be answered.

Our results are in agreement with those of Jabbarvand et al.[8], who evaluated the effect of mechanical implantation of MyoRing in 95 eyes of 95 patients with moderate and advanced KC and observed a significant improvement in UCVA and BCVA 1 month after surgery, which was consistent with a significant reduction in sphere (5.74 D) and cylinder (3.02 D). No significant changes were detected in these parameters afterward. Further, significant corneal flattening of a mean value of 9.78 D was obtained. Both spherical myopia and astigmatism showed reduction, but the reduction in myopia was more remarkable than in astigmatism. No significant change in central corneal thickness was observed at any point after the operation. There were no significant differences in visual gain between the two keratometry groups (higher or lower than 53 D) after the procedure. They concluded that MyoRing implantation has an acceptable efficacy profile in moderate and advanced KC.

A great advantage of the current MyoRing technology over ICRS is the postoperative access to all three theoretically possible degrees of freedom (implant thickness, implant diameter, and implant position) for achieving the best possible results compared with only one degree of freedom (implant thickness) in ICRS. In a previous study by Kamal et al., [21], Intacs SK (addition technology) was implanted in 22 KC eyes. They completed 6 months of follow-up. Two Intacs SK segments of 0.45 mm thickness were inserted into the cornea at the steepest axis, aiming to embrace the KC area to achieve maximal flattening. The mean UCVA significantly improved from 0.12 ± 0.13 to 0.38 ± 0.26. The mean BCVA also improved from 0.38 ± 0.28 to 0.62 ± 0.22. The Km reading improved from 51.32 ± 4.12 to 48.44 ± 3.92.

The following table shows a simplified comparison between different traditional treatment options as reported by Daxer International Keratoconus Center, Austria.

MyoRing could be considered a malleable line for KC treatment, as it can be safely combined with other therapeutic procedures, achieving additional beneficial effects. Behrouz et al.[23] combined MyoRing with Intacs for managing patients with advanced KC. They implanted a MyoRing in a patient who had undergone a previous Intacs implantation surgery 4 years previously without Intacs explantation and with significant residual refractive error. There were no intraoperative or postoperative complications. After 1 year, the mean keratometric power decreased from 50.3 to 43.6 D, UCVA improved from 20/400 to 20/50, and BCVA improved from 20/200 to 20/30. Daxer et al.[24] combined a new corneal crosslinking method with MyoRing implantation into a 'closed' corneal pocket through a narrow incision tunnel in one surgical session. Riboflavin is instilled into the corneal pocket without the need for epithelial debridement. A case of advanced KC treated in this manner is presented. UCVA increased by seven lines from 0.05 to 0.25, and the average central K-reading decreased by 11 D. The haze seen during the early postoperative period diminished in the first month after surgery.

Conclusion

MyoRing implantation into a corneal pocket through CISIS could be considered an efficient, safe, modifiable, and simple therapeutic option for KC treatment.